Porcine xenograft cardiac valves were first implanted in humans at the NHLI in July 1970. Since then 218 valves have been implanted in 177 patients as either single or multiple replacements. One hundred thirty-five patients have survived operation and have been followed for 3232 months. Early mortality is 12% and late mortality equals 13%. Anticoagulants are administered to patients with either a Starr-Edward or Bjork-Shiley prosthesis in another position. Four of 80 (5%) patients with isolated xenografts have had emboli; 2 in the early postoperative period and 2 late. Eight of the remaining 97 (8%) mixed valve replacements have had emboli anticoagulants. Death has not been attributed to thromboembolism in the isolated xenograft group. Hemodynamically the xenograft has been proven satisfactory in any cardiac valve position. Fifteen patients have been followed for periods greater than 4 years and five of these patients for more than five years. Prosthetic dysfunction occurred in one patient 57 months following implant.